Compared with the sea the surface of the land is rough and offers considerable resistance to the free passage of air masses. This is, of course, particularly true of western Cordilleran region, but Atlantic air masses also find difficulty in invading the continent beyond the Appalachians.
The Cordilleran region has a great effect upon the climatic pattern. The temperature variations due to elevation are alone enough to make it a distinct climatic region. There are two important results of its function as a barrier to the passage of air masses. In the first place, because the air must rise rapidly on reaching the mountains, with a great cooling and condensation of its moisture content, the West Coast is the area of greatest rainfall in Canada. Secondly, since this precipitation is withheld from the valleys and plains to the east of the mountains, the latter areas are correspondingly dry. Such areas in the lee of mountains are known as rain shadows and, in some parts of the earth, are occupied by extremely dry deserts.
The "chinook" is another striking effect of the mountain barrier. It is a warm dry wind which from time to time affects the area east of the mountains. In winter it may cause spectacular changes in the weather bringing temperatures of 50° F. or over to Calgary and Lethbridge in midwinter. In winter, as we have noted, the interior of Canada is occupied by cold Polar continental air while warmer air is found on the West Coast. The passage of a pronounced cyclone across the midwestern States and Great Lakes will cause the cold air to be drawn off rapidly to the southeast. Its place will be taken by the upper levels of the Pacific air which is easily able to flow across the mountain barrier and is greatly warmed by compression as it glides down the eastern slopes. Since this air has been elevated over the mountain ranges it contains little moisture, and its power of evaporation is greatly increased by the rise of temperature during its descent.
Its most visible result, therefore, is a rapid removal of snow cover from the Prairies. This is of considerable value to the ranching districts of southern Alberta where it makes winter grazing possible. Measured from this point of view, the area of a "chinook" rarely extends beyond Swift Current but the whole Prairie region experiences a spell of mild weather in sharp contrast with the "cold wave" which preceded it.